Data storage media commonly used for storage and retrieval of data comes in many forms, such as magnetic tape, magnetic disks, optical tape, optical disks, holographic disks or cards, solid state memory such as flash memory and the like. In magnetic media, data is typically stored as magnetization patterns that are magnetically recorded on the medium surface. The data stored on the medium is typically organized along “data tracks,” and transducer heads are positioned relative to the data tracks to read or write data on the tracks.
Servo patterns refer to patterns or other recorded marks on the medium that are used for tracking purposes. In other words, servo patterns are recorded on the medium to provide reference points to locate the data tracks. A transducer head detects a servo pattern and sends a signal representing the detected pattern to a servo controller. The servo controller interprets a detected servo pattern and generates a position error signal (PES). The PES is used to adjust the radial distance of the transducer head so that the transducer head is properly positioned to a data track for effective reading and/or writing of data to the data track.
Disc drives store data on one or more surfaces of one or more rotatable magnetic discs in data tracks in concentric circles or, alternatively, in spiral patterns. The data tracks are generally divided into a plurality of data blocks. Data blocks of a disc drive may also be referred to as sectors. In disc drives, servo patterns are generally recorded in specialized areas on the disc surfaces in amongst the data tracks, called “servo wedges.” Servo wedges serve as references for the servo controller. Some magnetic media include a plurality of servo wedges, with data tracks being located in data wedges between the servo wedges.
Even though disc drives include servo patterns to properly locate a transducer head during a write operation, data may be inaccurately located adjacent to the center of a data track. This is referred to herein as off-track write. Off-track write may occur due to vibration or servo error. One technique for recovering data recorded as off-track write is to perform off-track retry in the readback process. When off-track write is suspected at a data block, an off-track retry technique tries to off-track read the whole data block at various off-track positions. While more tedious than reading data that is accurately positioned in a data track, an off-track retry technique allows recovery of data that can not be read by looking only at the center of a data track.